Here is a sample "bottle surface" I created in a totally different
method, that also uses "Filled Surface" but includes a constraint
curve. Note that it uses a spline that is on another surface (it is
hidden, make sure to make it visible), but the ends are created
using "face curvature". This is done by selecting the edge, the
spline, and the face; you will then get the option to constrain
"face tangency" and curvature. You could also just use the middle
spline, and let SW figure out the rest.

Here is another variation, doing about the same thing. This
time I made a 3D sketch using convert entities on the edges, then I
split it into two curves. I then used boundary surface on each
edge, and went between them. This gives you far greater control
over the shape of the new surface than my previous example with
fill.

Looking good. I would like to get better at surfaces that why I
come to this board. I get stuck up when I am not able to do
surfaces better. I am going to look at what you did. I guess no
simple way to put a radius on the end then?